ci: add test.yml workflow for Node 18 and 20 matrix

[snipcodeit]

Summary

• Adds .github/workflows/test.yml — a dedicated CI workflow that runs npm test across Node 18 and 20 on every PR and push to main
• No GitHub token required; the test suite uses the mock layer exclusively and runs fully offline
• Each matrix leg reports as a separate named check ("Test / 18", "Test / 20") in the PR status panel

Closes #256

Milestone Context

• Milestone: v6 — Local Testing & Simulation Framework
• Phase: Phase 50: CI Integration & Coverage (issue 10/11 in milestone)
• Dependencies: Blocked by: Write scenario tests for mgw:sync drift detection and auto-sync #255 | Unblocks: Add vitest coverage reporting with 70 percent line coverage threshold #257

Changes

File	Change
.github/workflows/test.yml	Created — dedicated test workflow, 21 lines

Design Notes

The existing ci.yml already covers Node 18, 20, 22 with npm test + npm run build + lint. This test.yml is intentionally separate and minimal — it creates distinct named checks in the GitHub PR UI, fulfilling the requirement to "report test results as a PR check" under the Test workflow name. Only Node 18 and 20 are in scope per the issue spec.

Test Plan

• Verify workflow file exists at .github/workflows/test.yml
• Confirm workflow triggers on pull_request and push to main
• Confirm matrix includes Node 18 and 20 only
• Confirm npm test is the test command (no token needed)
• Confirm actions/checkout@v4 and actions/setup-node@v4 with cache: npm are used
• Observe PR check panel shows "Test / 18" and "Test / 20" checks once this PR is open
• All 387 existing tests pass on Node current (verified locally)